The strength distribution of OAM signals is described by Laguerre-Gaussian beam in the free space [1]}. In cylindrical coordinate systems, the Laguerre-Gaussian beam is expressed by [2]}
\({\begin{array}{c}u(r,\phi ,z) = \gamma \sqrt{\frac{{\mathfrak {L}!}}{{\pi \left( {\mathfrak {L} + \left| l \right|} \right)!}}} \frac{1}{{{w_l}(z)}}{(\frac{{r\sqrt{2} }}{{{w_l}(z)}})^{\left| l \right|}}{e^{ - {{(\frac{r}{{{w_l}(z)}})}^2}}}\mathbb {L}_p^{\left| l \right|}(\frac{{2{r^2}}}{{w_l^2(z)}}) \\\cdot {e^{ - i\frac{{\pi {r^2}}}{{\lambda {R_l}(z)}}}}{e^{i(\left| l \right| + 2\mathfrak {L} + 1)\psi (z)}}{e^{ - il\phi }} \\\end{array}} ,\qquad \mathrm {(7a)}\) 
\({R_l}(z) = z\left[ {1 + {{(\frac{{\pi w_l^2}}{{\lambda z}})}^2}} \right],\qquad \mathrm {(7b)}\) 
